Intumescence – when heat causes a protective layer to form

Foaming behaviour of passive fire protection materials

Not all fire protection materials react in the same way when exposed to heat. While ablative materials are designed to absorb heat and thereby cool, intumescent materials change in structure: they expand and form an insulating protective layer.

In three experiments, we show how varied this foaming behaviour can be. We look at the foaming factor, the resulting expansion pressure and the material’s structure. Andreas Pfitzinger explains the mechanisms behind the process and why the material properties need to match the specific fire protection application.

Expansion pressure: when volume turns into force

KuhnOdice: So foaming isn’t just about creating volume. What part does expansion pressure play?

Andreas Pfitzinger: Expansion pressure describes the force exerted by an intumescent material as it foams. It is important to note that expansion pressure does not occur to the same extent in every intumescent material; it depends on the formulation and the conditions under which the material expands. When a material is allowed to foam freely, without any external load, it primarily creates volume. However, if foaming is restricted, certain materials can build up mechanical pressure. This makes the resulting foam denser and more stable.

KuhnOdice: What is the main takeaway from the test setup?

Andreas Pfitzinger: Our experiments show that the material does not simply foam freely, but builds up against resistance. This again illustrates the force that can be generated during foaming. In practical terms, expansion pressure is particularly important where materials need to form a tight seal, overcome resistance or reliably close off an opening.

Expansion factor: how much volume is created?

KuhnOdice: Apart from the degree of pressure, the extent to which a material expands is also crucial. What exactly does the expansion factor describe?

Andreas Pfitzinger: The expansion factor describes how strongly an intumescent material expands when exposed to heat. The higher the expansion factor, the greater the volume that can be generated. This is particularly important when a specific gap needs to be bridged, a cavity filled or an insulating protective layer formed.

KuhnOdice: Does a high expansion factor automatically mean better protection?

Andreas Pfitzinger: No, not automatically. What matters most is the specific application. A high expansion factor can be helpful, but it is not always the solution of choice for every installation scenario. In confined installation spaces, excessive expansion can be quite problematic. That is why every material is designed for a specific area of application – its so-called “sweet spot”. Optimum protection is only achieved when expansion behaviour, installation space and fire protection requirements fit together perfectly.

Types of foam: not all intumescent materials behave the same way

KuhnOdice: If intumescent materials differ in terms of expansion factor and expansion pressure, do they also form different foam structures?

Andreas Pfitzinger: Absolutely. Depending on their chemical composition, intumescent materials can form very different foam structures. Silicate-based products form a very hard structure that develops under pressure and in a directed manner. Phosphate-based products, by contrast, generate a high volume but build up little pressure and therefore tend to expand more freely and in three dimensions. Graphite-based materials, on the other hand, can expand strongly while also developing high expansion pressure.

KuhnOdice: When to use which type of foam?

Andreas Pfitzinger: That depends very much on the type of application. If substantial volume is required but high pressure is to be avoided, a material that foams without generating pressure may be the solution of choice. But if, for example, an opening needs to be actively sealed or a component needs to be compressed, a material with appropriate expansion pressure is required.

Not all intumescent materials are the same

The key point about intumescent materials is not just that they increase in volume when exposed to heat, but how they do so. Expansion pressure, expansion factor and foam structure must all be aligned to suit the specific application.

While intumescent materials work by increasing in volume and forming a protective layer, ablative materials rely on energy absorption and cooling (through evaporation).

The expert behind the tests

Andreas Pfitzinger has been with the company since 2018, formerly known as svt Products and now operating as KuhnOdice. In his role as Product Manager, he combines sound technical expertise with an understanding of practical requirements.